Controlled combustion is generally performed for generating heat and/or power and typically takes place within a controlled environment, such as within an engine or other apparatus within a combustion chamber. Chemical reactants, often in a liquid or gaseous state are mixed in the combustion chamber forming a bulk gas ready for combustion. In a typical vehicular combustion engine, fuel and air comprising oxygen are mixed in the combustion chamber and compressed. The combustion process itself is generally initiated and maintained by heating the bulk gas to a temperature at which free radicals, such as for example O, OH, and H in the case of combustion of hydrocarbons, are formed to initiate dissociation and oxidation reactions.
The heat required to initiate the process typically originates from a localized source such as a spark. In the case of a standard vehicular combustion chamber, the spark is generated between the electrodes of a spark plug extending into a portion of bulk gases in fluid communication with the bulk gases of the combustion chamber.
It has also been shown in the last few decades that electrical discharges that generate non-thermal (non-equilibrium) plasmas can serve as an alternative and efficient way to produce radicals and promote combustion. One publication which describes this is Penetrante B. M. and Schultheis S. E., “Non-Thermal Plasma Techniques for Pollution Control”, NATO ASI Series G, Vol. 34, Parts A and B (1992).
Two well known ignition systems widely used are inductive discharge and capacitive discharge systems. These systems provide a single discharge spark suitable in most applications to initiate the combustion but are limited in influence on the combustion process.
Modern ignition systems aim for a controllable pattern of the discharge as disclosed, for example, in U.S. Pat. No. 6,729,317 to Kraus. Kraus describes how a high voltage switching polarity source should be used to drive the primary side of an ignition coil to produce spark discharge at high frequencies. Overall complexity limits the scalability and application of the system of Kraus.
The heat required to maintain the process after ignition typically is available from the combustion process itself. In a combustion process of hydrocarbon fuel and an oxidant (typically oxygen), since the chemical reaction is exothermic, as long as the conditions within the combustion chamber are appropriately controlled, such as the pressure and temperature of the unburned bulk gases, combustion of the bulk gases at the flame front generates enough heat to cause combustion of unburned bulk gas and propagates the chain reaction throughout the combustion chamber.
Complete molecular conversion during the process of combustion of pure hydrocarbons produces carbon dioxide and water. The chemical efficiency of this molecular conversion is dependent upon the generation and propagation of free radicals, which break carbon bonds. The generation, concentration, and propagation of these free radicals in turn depend largely upon the temperature of the bulk gases. To achieve sufficiently high temperatures for such conversion, a large amount of enthalpy is added to the bulk gases. These high temperatures may be achieved by direct heating, which as described above results from the exothermic reaction at the flame front, or a thermal electric arc which as described above may be used to initiate combustion.
The influence of electric discharge plasma on combustion processes has also been studied for several decades. Most of what is known about the effects of electric discharge plasma on combustion processes comes from studies of open flame combustion processes, and those studies strongly demonstrate improved stability, increased fuel efficiency and reduced emissions.
A class of known processes of initiating and maintaining combustion is described in “Method for igniting, intensifying the combustion or reforming of air-fuel and oxygen-fuel mixtures”, U.S. Patent Application Publication No. 2008/0309241 by Starikovsky. Starikovsky describes a process which, for reduction of ignition temperature and intensification of chemical reactions, includes the excitation of the combustible mixture in the combustion chamber by means of pulsed periodic nanosecond high-voltage discharges. According to Starikovsky, the discharge amplitude is set to maximize gas dissociation, and to prevent electron transfer into the whistler mode at the basic stage of discharge. Furthermore, as described in Starikovsky, high-voltage rise time is limited by the constraint of attaining uniform filling of the discharge gap with plasma and the effectiveness of the pulse energy transfer to the plasma. Starikovsky also describes how the high-voltage pulse duration is limited by the constraints of attaining a strong non-equilibrium character of plasma and the reduction of the discharge gap resistance.
Starikovsky's method uses monopolar discharge to produce plasma. A monopolar series of pulses, if unrestrained, can result in a continuous electric arcing, or equilibrium plasma, due to the remaining conducting medium in the discharge gap region. Therefore, the method of Starikovsky requires the additional constraint of ensuring there is a delay between the pulses that exceeds the plasma recombination time, i.e. a limited pulse frequency which is effective. For this reason, overall density of non-equilibrium plasma produced is limited, and during the time delay spanning the pulses plasma density may actually momentarily decrease, which acts to limit the improvement thereby provided to the combustion. Moreover, the method of Starikovsky may be ineffective in fast progressing periodic combustion such as that found in internal combustion engines. The technical implementation of nanosecond high voltage techniques also requires highly complex and costly equipment and has to provide the necessary high levels of electromagnetic radiation protection.
It would be advantageous to provide a system, circuit, and method for controlling combustion that mitigate at least some of the problems of the prior art.